This invention relates to a method and apparatus for implementing a skipping mode in power sources, and, more particularly, implementing a skipping mode which is programmable.
Switched mode power supplies (SMPS) are used in many applications in consumer and other products. Low power SMPS, for example are used in television receivers, personal computers, facsimile machines, video cassette recorders, and computer monitors. These SMPS are typically based on flyback transformer technology, but can utilize other technologies as well.
Because of the heavy use of these power supplies in high volume consumer applications, standby or idle power consumption is of great importance, since the devices are operated in idle mode much of the time. Additionally industry recommendations of the permissible idle power consumption levels of such apparatus are becoming increasingly stringent. Furthermore, again because of the high volume applications for these power supplies, low cost and compact construction are also significant factors.
In general, power supply efficiency is extremely low at low power levels. The use of a skipping mode allows the power supply to continuously operate at a high power level. This is accomplished by transferring higher power packets from time to time so that the average supplied power equals the load demand. The higher the power of the energy packets, the higher the efficiency.
Unfortunately, the so-called xe2x80x9cburstxe2x80x9d mode operation has one major drawback: the risk of audible noise in the apparatus being powered. The most effective way to reduce this audible noise is to limit the peak current in this mode. The packet energy level must then be limited. Thus it is useful to have the capability of adjusting the skipping threshold, that is, the power demand threshold at which a new power packet is to be supplied to the load, as this allows obtaining the best compromise between low standby loss and acceptable noise in any application, since different applications have different tolerances for noise from the power supply.
SMPS generally work in either voltage mode or in current mode. The regulation of the output voltage and skipping function is controlled by an integrated circuit controller which controls the action of a power transistor in transferring energy form the primary inductor of a transformer to the output. In voltage mode, an error amplifier output controls a power switch on-time. In this case current cycles are skipped when the demanded on-time is lower than a certain level. The peak current varies with respect to the input voltage as follows:
xe2x80x83Ipk=Vin*ton/Lp
where: Ipk is the peak current
Vin is the rectified input voltage
Ton is the on-time
Lp is the transformer primary inductance
Therefore the peak current is proportional to the input voltage. If the skipping mode is implemented in a voltage mode controller, the peak current corresponding to the skipping threshold is proportional to the rectified input voltage.
In current mode, the error amplifier directly controls the peak current. In this case, current cycles are skipped when the peak current required by the error amplifier is lower than a chosen level. Consequently, the skipping threshold determines the skipping peak current. The current mode is, then, the operation mode that best fits the skipping mode technique, but the invention can also be used with a voltage mode circuit.
In both cases it is important to be able to program the skipping threshold. This feature is easy to perform using an additional pin of the controller dedicated to this function. Unfortunately, the pin count in integrated circuits, particularly where cost or size is a consideration, is very critical and no pin may be available for such a function.
It would be desirable, then, to be able to combine the skipping peak current adjustment with the current sense function such that both functions may use the same input pin on the integrated circuit.